Week 6: Image Acquisition CR and DR

Question 1

Analog to Digital Conversion (ADC)

Answer 1

1. analog voltage values of the x-rays exiting the patient must first be sampled
2. the sampled data is then given a numerical value, this is quantification
3. once quantified and assigned a binary digit the signal can be considered digital
4. the number are then associated with a specific colour of grey based on their value

Question 2

Histogram

Answer 2

Graph that uses vertical columns to show frequency. It record how often a certain amount of attenuation occurred. To the right are tissues that attenuate less, to the left are tissues that attenuate more. EI number assigned here.

Question 2

Rescaling

Answer 2

The imaging software can recognize the characteristic curve even with a bad exposure and will rescale it to try and produce something in the diagnostic range.

Question 3

LUT

Answer 3

Look up table. The system has an ideal contrast and brightness scale for each body part in the computer and it is attached to each anatomical program. The system will compare the data generated by the image to the LUT and make adjustment to try and give you the ideal image.

Question 4

EI Numbers

Answer 4

Exposure indicator. Gove information about how many photons actually hit the IR. Should be evaluated after every exposure to help quality control of images and radiation protection.

Question 5

Detective Quantum Efficiency

Answer 5

Measures how well a system can convert its incoming attenuation data into image data. DQE is a measure of the information transfer efficiency of an IR from 0 to 1. 1 = data converted perfectly. Higher DQE typically means higher quality images at a lower dose.

Question 6

4 steps involved in creating visible image from a PSP (CR)

Answer 6

1. Expose: x-rays provide energy to be stored
2. Stimulate: red laser releases the stored energy
3. Read: blue light is emitted then gathered/amplified
4. Erase: white light resets the IP for reuse

Question 6

Direct Conversion DR

Answer 6

Incident x-rays are absorbed by photoconductors in the image acquisition device then immediately converted into n electrical signal all in one step.
1. incident x-ray data exiting the patient strikes the IR
2. Top electrode: just prior to the x-ray exposure a high voltage charge is applied to the top of the surface of the amphorae selenium (a-Se) layer. When the x-rays ionize the selenium will free electrons for collection at the bottom of the detector element.
3. Amorphous Selenium: a-Se is a semiconductor that is really good at detecting x-ray photons
4. Storage Capacitor: stores the generated electron charge and transfers this data to the TFT
5. TFT: can be thought of as a switch, the TFTs gathers the electrical information and reads it out line by line to the compute for processing.

Question 7

Indirect Conversion DR

Answer 7

Incident x-rays are converted to light by a scintillator in the image acquisition device before being converted to an electrical signal, this is a two step process.

Question 8

Indirect DR with Photodiode

Answer 8

1. Incident x-rays: attenuated x-ray data exiting the patient strikes the image receptor.
2. Scintillator: changes x-rays to light. Cesium Iodide (CsI) is most commonly used because it can be manufactured in needle like crystals which decrease light spread, helping to channel the light toward the detector elements.
3. Amorphous Silicon Photodiode: a photodetector which is a substance that is good at turning light into an electrical signal. A-Si detects the Leith created by the CsI scintillator and conducts it to the storage capacitor and TFT.

Question 9

Indirect DR with CCD

Answer 9

1. Scintillator: the same needle like CsI phosphors are used to receive incident x-rays which cause them to emit light.
2. Optic Coupling: series of either lenses or fibre optic cables which couple the phosphor to a sensor chip.
3. Sensor Chip: chips are able to convert light signals to an electrical signal and send it to the computer for processing, eliminating the need for the TFT array.

Question 10

DEL

Answer 10

Detector Element. Every individual pixel in the image matrix is associated with a DEL which allows for very precise data to be taken. DELs make up the active matrix array (AMA) sometimes called TFT array. The larger the sensing area on the DEL = larger fill factor which increases spatial resolution and contrast.